> If we're eating as many calories, or slightly more, as our grandparents did, while barely moving in comparison to them...

So you don't think that people who do more work are "supposed to be" more hungry and thereby eat more food? The people I know who are leaner seem to just not get hungry as often--or as much--as I seem to get... and hell: I've gone through periods where I'm gaining weight and periods where I'm losing weight, and when I'm gaining weight I feel like I'm hungry constantly and when I'm losing weight I find eating a bit tedious.